A valuable class of photographic supports and elements comprises a paper base material having thereon a polyolefin coating containing a white pigment and an optical brightener. Such supports are particularly useful in the preparation of photographic elements such as color prints because they exhibit good brightness and excellent dimensional stability and are highly resistant to the action of aqueous acid and alkaline photographic processing solutions. The polyolefin coating provides a very smooth surface which is desirable when thin layers, such as silver halide emulsion layers, are to be coated thereover, U.S. Pat. No. 3,411,908 describes such a support which has achieved widespread commercial acceptance.
The purpose of the optical brightener is to make the white areas of the support appear even brighter. The optical brightener fluoresces upon irradiation with UV (ultraviolet) light, emitting visible light, usually bluish in hue, thus enhancing the brightness of the support. Optical brighteners for use in photographic print materials must absorb UV light, especially in the region from 360 to 420 nm, and reemit such light so as to enhance the brightness of the print, and have the desired brightening power. The optical brightener must also be stable to the temperatures, as high as 310.degree.-330.degree. C., used in incorporating it into the polyolefin and in extruding the polyolefin onto the paper base material.
Moreover, the optical brightener must be nonmigrating so that it remains in the polyolefin coating and does not exude as a surface film on the polyolefin. Such exudation not only can give rise to a nonuniform brightness of the reflection surface of the support, but also readily transfers to any other surface contacted with it. For example, brightener transferred nonuniformly to the back side of the adjacent layer of support when wound in roll form can adversely affect subsequent coating and finishing operations and, in consequence, the quality and performance of the final element.
U.S. Pat. No. 3,501,298 describes a photographic element having a support comprising a paper base having thereon a polyolefin coating which contains titanium dioxide and bis(alkylbenzoxazolyl)thiophenes.
U.S. Pat. No. 3,449,257 relates to compositions comprising hydrophobic polymers and nonmigrating optical brighteners and to paper supports coated with such compositions. The nonmigrating optical brighteners are 2,5-bis(benzoxazolyl)thiophenes.
U.S. Pat. No. 3,260,715 discloses fluorescent bis(benzoxazolyl)stilbenes, such as 4,4'-bis(benzoxazol-2-yl)stilbene, which are useful as fluorescent brightening agents for textile fibers, papers, resins and photographic color print materials.
However, these and other prior art brighteners do not exhibit the combination of absorption/emission characteristics and brightening power, heat stability, and resistance to brightener exudation to the levels desired for photographic supports and elements. Thus, what has been desired is a photographic element comprising an optically brightened support, such support having improved resistance to brightener exudation and wherein the brightener exhibits excellent absorption/emission characteristics, brightening power and heat stability.